1. Field of the Invention
This invention relates to electrostatic precipitation and, in particular, to an improved electrostatic precipitation apparatus and method for removing suspended particles from gases whereby the efficiency of particle removal is improved and particle re-entrainment in the effluent gases is reduced.
2. Description of the Prior Art
Conventional duct type electrical precipitators for removing suspended particles from gases are provided with collecting plates on which suspended particles are precipitated due to action of an electrical field as the gases flow past the plates. It is known in the art to provide the collecting plates with openings so that the particles attracted to the plates can pass through the openings and be collected in the relatively dead space on the other side. U.S. Pat. No. 1,926,025 is representative of a device of this type. It has also been proposed to withdraw some gas through the collecting plate openings in an effort to keep the openings in the plates from clogging up. U.S. Pat. No. 1,769,338 discloses this concept. Other proposals have included combinations of electrical precipitation and bag type filters in which the fabric of the collector bag may be of electrically conductive material. In this type of system all of the gas is passed through the fabric and the particles are filtered out on one surface of the fabric. U.S. Pat. No. 3,839,185 is representative of such system. Each of these systems has attendant problems.
In the non-filter collector plate type electrical precipitator, particle re-entrainment has been a major problem.
Particles once captured on the collector plate do not necessarily remain captured. Solid particles are, in many cases, only lightly held to the electrode surfaces and can be easily dislodged by the windage effect of the gas flowing through the precipitator. This re-entrainment of precipitated particles is referred to as erosion. The term erosion usually includes all ways by which collected material can be lost through re-entrainment due to gas motion. The most important erosion effects are:
1. Direct scouring action of the gas on the collected dust on the electrodes, PA1 2. Carry-through by windage of dust falling from the electrodes, this dust being initially loosened by its own weight or by rapping of the electrodes. PA1 3. Sweepage of dust directly from hoppers caused by poor gas flow conditions or by air ingress into hoppers. PA1 1. Higher through-put velocities may be used and/or less dependence on maintaining uniform velocities throughout the precipitator. PA1 2. The need of maintaining an appropriate corona current through the dust cake for dust retention is replaced by the pressure drop force. Elimination of the need for corona current in the cake in turn eliminates the dependency of the process on dust resistivity, either high or low. Corona current can be eliminated in the collector by using a dust precharger, together with a non-corona collecting electric field.
Erosion in precipitators is usually a combination of these three effects. The gas velocity itself is, however, the most important consideration and erosion is usually found to set in rather suddenly as precipitator gas velocity is increased. Erosion is a function of the dust being precipitated, of the configuration of the collecting electrodes, of the gas velocity distribution in the precipitator, and of the degree of turbulence and eddying of the gas in the precipitator.
Dust loss due to erosion can be visually observed or monitored with a photoelectric smoke-density recorder. Considerable "puffing" is usually observed at high boiler loads. "Puffing" is the phenomenon whereby the precipitator loss is irregular in nature being quite light at one instant and quite heavy a fraction of a second later, i.e., the ash loss appears to be concentrated in sporadic bursts lasting from a fraction of a second up to a few seconds.
Particle loss by re-entrainment is one of the most severe and oft recurring limitations present in the electrostatic precipitation of dry particles. Reentrainment is especially important when one considers the requirements of modern high efficiency precipitators where the loss of only a few percent of collected particles is sufficient to spoil performance.
The fundamental precipitator factors which may either cause or prevent re-entrainment include gas velocity and measures to insure uniform, low turbulence gas flow. Light, fluffy particles which are easily re-entrained and settle slowly in the gas generally limit gas velocities to a maximum of three to five feet per second. On the other hand, particles which form dense, compact layers on the collecting electrodes may be collected at much higher gas velocities of 10 ft./sec. to 15 ft./second. However, with poor gas flow conditions, these precipitation velocities may have to be reduced by a factor of two.
Re-entrainment is sensitive to corona voltage and current, as well as to voltage wave shape and precipitator sparking. Many industrial dispersoids have electrical resistivities in the bulk collected state of the order of 10.sup.8 ohm-cm to 10.sup.10 ohm-cm, which is sufficiently high to cause substantial attraction forces to the collecting electrode when permeated by the corona current, although not high enough to induce back corona. Under these conditions, the adhesion of the collected particle layers is greatly enhanced by the flow of strong and well distributed corona currents through the layers.
The magnitude of this electrical attractive force depends on the particle resistivity and the corona current density. The dust experiences a force proportional to the electric field strength, which in turn is proportional to the corona current and to the dust resistivity. Thus, the binding force increases both with resistivity and with current density, so that the resistivities which approach the critical value of about 10.sup.10 ohm-cm are helpful in holding the collected particles.
Precipitator sparking produces particle resuspension in two ways. First, the corona currents are interrupted for the small fraction of a second during which the spark occurs and some small portion of the collection is lost. Second, the spark itself locally disrupts the dust layer on the electrode and literally forms a small "bomb crater" and a local dust explosion. At high sparking rates these effects are multiplied many times per minute, and resultant dust resuspension becomes serious.
Rapping clearly may have a profound effect on re-entrainment. Excessive rapping tends to re-entrain all of the dust collected, and insufficient rapping leads to heavy dust buildup on the plates with resultant poor electrical operation and large re-entrainment losses.
Particles of low resistivity are especially vulnerable to re-entrainment because the corona-current binding forces are non-existent or even negative. Such particles may be actually repelled from the collecting surface owing to the pith-ball effect. The repulsive effect is very noticeable in the case of gritty particles which originate from poor combustion of the pulverized coal. These particles are relatively large, typically 100.mu.to 200.mu., and have low density, and low resistivity of the order of 10.sup.4 ohm-cm.
Particle deposits on the collection surfaces of a conventional precipitator must possess at least a small degree of electrical conductivity in order to conduct the ionic currents from the corona discharge to ground. The minimum conductivity required as shown both by theory and experience is about 10.sup.-10 inverse ohm-cm. Particles having conductivities less than the critical value of 10.sup.-10 are referred to as high resistivity particles, the critical minimum value of resistivity being about 10.sup.10 ohm-cm.
In precipitator operation high particle resistivity is usually manifested by disturbed electrical conditions in the form of excessive sparking at moderately lowered voltages or by excessive current at greatly lowered voltages. These effects in turn cause loss of precipitator efficiency, the loss in performance increasing with resistivity. When resistivity exceeds about 10.sup.11 ohm-cm, it becomes very difficult to achieve reasonable efficiencies with precipitators of conventional design. Special types of precipitators must then be used or measures taken to reduce resistivity.
Fly ash collection comprises more than half of the total precipitator installations in terms of gas treated. Fly ash is a generic term used to designate the particulate matter carried in suspension by the effluent or waste gases from furnaces burning fossil fuels. In modern usage, the term usually refers to the particulate omission from the burning of pulverized coal. The character and properties of the ash, including resistivity, vary widely with such factors as the coal burned, design and operation of the furnace, and the steaming rate of the boiler. Not only may the ash differ greatly from plant to plant, but may also very from day to day in a given plant.
Major constituents of most fly ashes are silica, alumina, and iron oxide. The first two are present primarily as silicates, which give fly ash particles their typical glassy appearance. Carbon may also be a major constituent of some fly ashes, ranging from a fraction of a percent for good combustion up to 40% or even 50% for very poor combustion. A carbon content of about 10% or greater usually is sufficient to ensure low resistivity of the ash. There is also a water soluble portion of fly ash which, although usually only a few percent or less, is of great importance in determining the electrical conductivity of the particles.
Measurements made on many fly ashes under actual field conditions show normal values of resistivity below the critical value of about 10.sup.10 ohm-cm, some in the marginal zone between 2.times.10.sup.10 ohm-cm and 5.times.10.sup.10 ohm-cm where precipitator trouble is probable and a few in the region about 5.times.10.sup.10 ohm-cm where trouble is certain. Inasmuch as the moisture content of practically all boiler gases lies in a narrow range of 6% to 9%, it is evident that moisture is only a minor factor in the wide variations observed for fly ash resistivity. Carbon is also a minor factor for the great majority of fly ashes.
Back corona is the descriptive term for the local discharge from the normally passive electrode in a corona-discharge system when the electrode is covered with a poorly conducting dust or fume. Under suitable conditions of corona voltage and current, the layer breaks down locally and a small hole or crater is formed from which a visible back corona discharge occurs. Such discharges reduce precipitator collection efficiency by lowering sparkover voltage and by producing positive ions, which decrease particle charging.
In a corona discharge system with a dust layer deposited on the passive electrode, if the dust is a good conductor, there is little or no disturbance of the corona discharge. However, as the dust conductivity is decreased, a point is reached where the corona ions begin to be impeded by the resistance of the layer. This causes the voltage to increase across the layer and to correspondingly decrease across the gas, with the result that the corona current falls somewhat. As the dust conductivity is further reduced, the voltage across the layer continues to increase and finally causes dielectric breakdown of the layer. This is the onset point of the back corona discharge. Depending on conditions, the localized breakdown of the dust layer may either propagate across the corona gap and thus cause a spark, or remain localized and form a stable back corona crater. Stable back corona is marked by the appearance of one or more blue colored local discharges on the dust layer. In severe cases, the dust layer will be covered with literally hundreds or thousands of such glow points per square foot. The corona at the wire then becomes concentrated into a relatively few intense stationary brushes of somewhat white appearance, and the corona current rises to several times the normal current. Formation of back corona craters is especially favored by thin dust layers and high resistivity dust. Severe back corona has been observed with dust layers as thin as 0.1 mm, but a dust layer a little over one particle thick can reduce the sparking voltage by 50%.
Development of practical means for overcoming or circumventing high resistivity effects in electrical precipitation has long been a major goal. Early endeavors used moisture and acid conditioning. Earlier investigators also tried brute force methods including moving belt electrodes, rotating brushes, and various other gadgetries. These were uniformly unsuccessful not only because of the troublesome mechanical problems introduced, but also because most of the schemes are unsound, in that even thin films of dust can produce severe back corona effect.
High resistivity problems may be avoided by the use of water flushed or wet film collecting electrodes. The wet film principle has been applied successfully on a pilot scale to two stage precipitators in which the water film is used only in the charging section and the collector section operates dry. Dusts have resistivities as high as 10.sup.12 or 10.sup.13 ohm-cm have been collected at high efficiency by this method in test units of 1000-cfm or 2000-cfm capacity. However, the problems of maintaining the water films over long periods of time have prevented large scale use of this method so far.
Another approach which has been effective in laboratory tests is based on temperature control of the collecting electrodes. Either cooling or heating may be used to shift the electrode temperature out of the critical intermediate range near the peak of the resistivity curve. Heating a large electrode surface to the required high temperature region necessitates large amounts of power; on the other hand, cooling the electrode may well result in condensation and fouling of the electrode surface by wet dust deposits.
More recent work has included the use of chemical additives to adjust the resistivity of the collected dust. Some success has been demonstrated, however, the variabilities of fly ash characteristics as previously noted and the economics of additives have limited the widespread use of this approach.